1. Field of the Invention
The present invention relates to a device for separating transmitting waves and receiving waves, especially to a device provided in radio communication equipment, the equipment being used in a system in which a transmitting frequency band and a receiving frequency band are different from each other and the equipment transmitting and receiving signals simultaneously, for separating transmitting signals and receiving signals.
2. Description of the Related Art
These days the number of people carrying mobile communication terminal equipment such as cellular phones and portable data processing devices is increasing. The mobile communication terminal equipment usually performs communication with other terminal equipment by establishing a channel on a network in a specified radio frequency band, and transmitting and receiving radio signals over the channel. Therefore, such mobile communication terminal equipment needs a function for separating transmitting and receiving waves.
FIG. 1 explains the separating of transmitting and receiving waves. Here, for example, the separating of transmitting and receiving waves in mobile communication terminal equipment is explained. The mobile communication terminal equipment shown in FIG. 1 transmits signals generated in a transmitter unit 103 to a network through a common port using a transmitting wave. At the same time it obtains receiving signals out of a receiving wave received from the network through the common port and processes them in a receiver unit 104. Here, as shown in FIGS. 2A and 2B, a frequency band for a transmitting wave (transmitting band) and that for a receiving wave (receiving band) are different from each other.
A separating device 100 is provided with a receiving filter 101 and a transmitting filter 102. As shown in FIG. 2A, the receiving filter 101 is a band pass filter which passes only frequency elements for the receiving band and attenuates those for other than the receiving band. Therefore, when waves received from the network passes through the receiving filter 101, frequency elements other than for the receiving band are substantially eliminated and only signals of the receiving band remain. Then, the receiver unit 104 obtains transmission signals from signals which have passed through the receiving filter 101 and reproduces them.
On the other hand, when transmitting signals generated in the transmitter unit 103 are sent out to the network over a transmitting wave in such a common port configuration as shown in FIG. 1, part of the transmitting wave is input to the receiving filter 101. This transmitting wave also contains a frequency element for the receiving band (this is usually contained as noise). Because, generally speaking, the transmitting radio power is far greater than the receiving radio power, the frequency element for the receiving band is not eliminated by the receiving filter 101 and is input into the receiver unit 104 if it is contained in a transmitting wave. Accordingly, there is a fear that it becomes difficult or impossible to reproduce the signals transmitted over a receiving wave or the receiver unit 104 may be saturated. For this reason the separating device 100 is provided with the transmitting filter 102, which is a band elimination filter, for sufficiently attenuating the frequency element for the receiving band contained in a transmitting wave. In other words, by introducing the transmitting filter 102, the frequency element for the receiving band is eliminated from the transmitting wave.
Incidentally, in a mobile communication system usually a great many (several hundred to several thousand) communication channels are provided within a communication band allocated to each common carrier. When a mobile communication terminal communicates with another terminal, a channel in an unused state (idle channel) out of these many communication channels is allocated to a connection. Thus, each mobile communication terminal can receive signals of all frequencies within the receiving band and transmit signals of any frequency within the transmitting band. Namely, as shown in FIG. 2A, receiving filter 101 passes the entire receiving band, and as shown in FIG. 2B, transmitting filter 102 eliminates the entire receiving band. In other words, the pass band width of receiving filter 101 is broader than the entire receiving band, and the elimination band width (attenuation band width) of transmitting filter 102 is broader than the entire receiving band.
The separating device 100 can also be provided with a band pass filter for passing transmitting bands only in place of the elimination filter as the transmitting filter 102. However, the attenuation volume of signals in a transmitting band is smaller if a band elimination filter which eliminates the receiving band is used instead of a band pass filter which passes the transmitting band. For this reason, as a transmitting filter provided in a separating device, especially in the case that it is used in a radio communication terminal where compactness is required, a band elimination filter which eliminates receiving band is mainly adopted.
Generally, a communication band width in a mobile communication system is relatively broad, such as several MHz to several tens of MHz. For this reason, a broad elimination band which covers the entire receiving band and a broad pass band which covers the entire receiving band are required for a transmitting filter and a receiving filter comprising a separating device. To separate a transmitting wave from a receiving wave, each of transmitting and receiving filters must provide a large amount of attenuation (in the case of a resonant type filter, a high resonant frequency Q).
However, broadening a band width and increasing an attenuation volume conflict with each other. For this reason, to make a filter provided with both broad band and large attenuation, several filters with different frequency characteristics (in the case of resonant type filters, with different resonant frequencies) have been combined. Many transmitting and receiving filters of the existing separating device are composed of three to seven stages of resonant type filters.
In this way, when several resonant filters are combined, it becomes difficult to miniaturize a separating device. Naturally, its weight also increases. Furthermore, its material and processing costs increase, because the number of components is larger.